SUMMARY Globally, influenza virus is a major public health burden. Evidence suggests that an individual?s earliest exposures to influenza have profound effects on immunity against the virus throughout life. Well-curated, infant cohorts, used to interrogate the full spectrum of immune responses against early influenza exposures, and examined for subsequent exposures have been lacking. Our premise of this proposal is that the antigenic form (strain, infection vs. vaccination) of an encounter with influenza virus results in disparate immunological memory profiles, particularly in B and T cell repertoires. Specifically, we will examine how influenza infection- and vaccination-mediated imprinting modulates the breadth, longevity, and functional quality of B cell and antibody responses and dictates the specificity, receptor profiles, functional activity and epigenetic state of the T cell responses to subsequent influenza exposures. In addition, we will examine how these parameters are impacted by repeated exposure to influenza. We will address these questions by comprehensively assessing the B and T cell immune responses induced by influenza infection and vaccination in three unique birth cohort studies: 1) Managua, Nicaragua; 2) Wellington, New Zealand; and 3) Los Angeles, CA. Through the three sites, chosen because of our extensive experience with conduct of cohort studies and enrollment of infants, distinct seasonal transmission of influenza, and diverse demographics, we will establish a cohort of ~3,100 children, with 2,100 newborns enrolled over the course of the U01 and an additional ~1,000 children who have already been followed since birth for up to 8 years for symptomatic influenza infection and have existing banked blood samples. Our central hypothesis is that the chronology of initial and subsequent influenza exposures has long lasting effects on anti-influenza responses (?imprinting?), setting infants on diverse immunological trajectories mediated by the recruitment of distinct B and T cell specificities and functional capabilities. We will test this hypothesis through establishment of the cohort (Aim 1), examining the response to initial exposures (Aim 2), examining the response to subsequent exposures (Aim 3) and evaluating correlates of protection (Aim 4). We have formed a Consortium of world-renowned investigators who have a long history of collaboration (>150 publications). Combining extensive experience and on-going programs in clinical, immunological, and virological research ensures a high-quality, successful research program. This U01 will result in: 1) New insight on the mechanisms of imprinting; 2) Improved understanding of what constitutes protective immunity in early and subsequent influenza infections, including the effects of vaccination; and 3) Identification of natural and vaccine- induced B cell/antibody and CD4+/CD8+ T cell correlates of protection. These investigations provide a novel opportunity to combine unique cohorts with exhaustive immune profiling, generating critical insights for new vaccine strategies to induce broadly protective immunity to influenza virus.